It is well known that the optics of the human eye limit many aspects of visual function including visual acuity and contrast sensitivity. Less is known about how the eyes? optics affect binocular vision. There is clear evidence that optics affect both stereopsis and binocular summation. Interestingly, inter-ocular differences in optics impede some aspects of binocular function while facilitating others. Moreover, our previous studies found that in eyes with prolonged visual deprivation induced by the abnormally large optical defects such as aberrations, neural processing of both contrast and phase of retinal images are altered. Our preliminary studies have also shown that binocular visual functions e.g. binocular summation, subjective image quality and stereo-resolution are determined by strong binocular neural interaction with the optical profiles of the two eyes. We are now ready to distinguish contributions of optical and neural factors to binocular vision, including in patients with the corneal disease called keratoconus (KC). With this unique condition, a normally developed visual system suffers optical degradation in adulthood, thus providing opportunity to study adaptation to induced optical aberrations on binocular visual function. We will utilize innovative advanced correction tools, a binocular adaptive optics vision simulator and a customized scleral lens for short-term and long-term precise aberration correction/manipulation to study (1) the effects of optics on binocular combination of dissimilar monocular images, (2) contributions of binocular optics to human stereopsis and (3) binocular neural plasticity stimulated by improved optics and binocular perceptual learning. Aim 1 is designed to investigate the mechanisms that underlie binocular combination of dissimilar retinal images induced by inter-ocular differences in the eye?s optics. This is achieved by testing the hypothesis that the human visual system can achieve binocular combination of neural signals arising from left- and right-eye images in a manner that enhances the global binocular image quality via interocular processes operating over local regions of visual space in an eye-selective manner (1.1), by characterizing changes in sensory dominance induced by long- term adaptation to eye?s optics (1.2) and by testing the hypothesis that dissimilar monocular aberrations alter phase perception of a broadband stimulus differently in the two eyes, resulting in reduced binocular advantages and inter- ocular inhibition in some severe cases. Aim 2 is to investigate both optical and neural factors including aberrations, fixational eye movements and long-term neural adaptation in human stereopsis. Aim 3 will assess the extent to which binocular plasticity that occurs during long-term adaptation is reversible and what mechanistic changes underlie this reversal once aberration-free image quality in the two eyes is achieved in KC eyes. Successful completion of these specific aims will produce fundamental insights into human binocular vision, specifically the relationship between ocular optics and the binocular neural adaptive process.